In the course of drilling oil wells, there have been occasions when being able to rotate the upper part of the drill pipe string while the lower part is stuck or stationary would be desirable. Being able to rotate the upper string while jarring on the stuck lower string would tend to keep the upper string from getting stuck. Disengaging the upper and lower segments of the drill pipe string can be accomplished through a clutch or coupling, which may be operated from the surface, or by a torque limiting device that would slip to keep from twisting off the lower string when it gets stuck or over-torqued.
Those who are schooled in the art can design both of these devices. The problem is designing a rotary thrust bearing that will operate under the high loads imposed by the long drill string.
In addition to the load imposed on the thrust bearing by the drill string itself, there are additional axial thrust loads imposed. For example, many times when drilling hard formations, the drill bit bounces off the bottom of the hole. This sends vibrations up the string that add to the thrust bearing load. In addition, when jarring, the tension and release of the jarring action on the drill string increases the load by as much as three times the static load.
Traditionally, designers have used ball bearings or roller bearings for this service. However, due to the restricted outside diameter imposed by the hole size, the bearings have been stacked in parallel to achieve the required capacity. Stacking in this manner requires some provision for the bearings to equally share the load. This has been done by using springs and elastomers with varying success.
Problems associated with the use of ball or roller bearings generally occur not when they are rotating but when they are not rotating. For example, the constant vibration of drilling can cause the balls or rollers to brinell the races in one spot. Brinelling is the process of making a small dent in the race where each ball or roller rests. This causes the bearing to run rough when it does rotate, reducing the life of the bearing. Brinelling also causes cracking and breaks off small portions of the brittle balls and races which further reduces the useful life of the bearing.
A hydrodynamic bearing eliminates the brittle balls or rollers and uses a fluid such as oil to keep the races separated. Previous fluid bearings, such as the one described in U.S. Pat. No. 5,286,114, use an external fluid and pressure source. Bearings of this type depend upon a thin fluid film for friction reduction and a seal. However, the bearing loses a considerable amount of fluid through this seal, which must continuously be refreshed from the external source. Moreover, extremely close tolerances are required to maintain the same film thickness across each pair of races.
The present thrust bearing invention provides the required axial thrust bearing capacity to allow a drill pipe string to be separated into rotating and non-rotating sections and absorb the imposed thrust loads. A clutch, torque limiter, or other release mechanism is located in the string between the thrust bearing and a section of drill pipe string to selectively prevent torque transfer between the sections of drill pipe string and thus permit relative rotation. The thrust bearing allows an upper or lower drill string section to rotate relative to the other non-rotating section. While doing so, the thrust bearing invention eliminates the continuous outside pressure and fluid source requirement, and reduces the extreme tolerance requirements of previous hydrodynamic bearings.
The thrust bearing of the current invention contains a series of alternating relatively high and low pressure chambers (at least two) defined by pairs of races, or pressure chamber segments, journalled between a housing and a mandrel. An axial bore allows fluid flow through the thrust bearing.
Each pressure chamber segment contains one or more seals, and additional seals may be located in the races, in the housing, in the mandrel, and/or in the other elements to reduce the leakage of fluid and loss of pressure associated with prior hydrodynamic bearings. All of the relatively high pressure chambers are ported to a common fluid reservoir, allowing communication of the relatively high pressure fluid and equalization of the pressure throughout the chambers. Similarly, the relatively low pressure chambers may be ported to a common low pressure external fluid, or to a common low pressure fluid reservoir.
The high and low pressure chambers are held in slidable relation within the thrust bearing. The higher pressure fluid is confined to a given volume and is ported to a space between each pair of races which form a single pressure chamber. When a load is applied to the bearing it tends to reduce the space between the pair of races, thus increasing the fluid pressure. Since the high pressure fluid is ported to a common reservoir and each pair of races, the total load capacity of the thrust bearing invention is the sum of the loads carried by each pair of races. Accordingly, the capacity of the thrust bearing may be increased simply by adding pairs of races (pressure chambers) to the bearing. In addition, because the thrust bearing is comprised of a multitude of pressure chambers in a constant diameter tubular mandrel, it provides a fluid bearing at a size compatible with drilling operations.